this may not be a popular opinion here, but at some point management has to set some precedence and draw the line in the sand.
personally, I would say:
"look, the deadline is 3 days before training camp. if you want to play for the Leafs this season (or ever again), here are your options:
1 - $X AAV for Y years
2 - $A AAV for B Years
3 - $C AAV for D Years
pick which of the three options you prefer, but if we go past the deadline, we're pulling the offer and you will either sit for the year or be traded. Feel free to explore offer sheets but make SURE it's a stadium that you'd actually want to play 41 games in."
If Dubas set this as an ultimatum, I'd give him mad respect and would not feel the least bit slighted if Marner & Co. chose to leave his teammates and team for pastures elsewhere. You can leverage and negotiate all you want, but when push comes to shove, you can't have your cake and eat it too. If you have too much personal pride and desire to max your contractual returns, it's your prerogative - just don't be too surprised or upset if you end up in Carolina, Arizona, or on the outside of the NHL and looking in in 2019-2020.